### source function

**(S)**
(measure indicating the effect of a material on EMR passing through)

A **source function** (often indicated as **S**) is a measure of the
effect of a material (e.g., gas or plasma) on electromagnetic radiation
(e.g., visible light) passing through, specifically how much is
added (emission coefficient) divided by how much is removed
(absorption coefficient). It has the same units as
intensity, and the intensity of a beam of light tends to
change toward and approach the source function, virtually matching
it if the beam passes through sufficient distance with a constant
source function. An equation of radiative transfer may be written in terms of a source
function:

1 dI_{λ}
- ——— ——— = I_{λ}-S_{λ}
κ_{λ}ρ ds

- λ - wavelength
- κ - absorption coefficient
- s - position along the beam, i.e., ds is for distance traveled
- I - intensity
- ρ - density

The **Eddington-Barbier relation** relates the source function to
the flux leaving a star, indicating it matches the source function
at a vertical optical depth of 2/3, which is a basis for
defining a photosphere as beginning at that depth.

(*physics,EMR,measure*)
http://en.wikipedia.org/wiki/Source_function

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